Refining current design methods for vertical borehole heat exchangers considering complex ground conditions

• Author / Creator

  Guo, Yunting

• Ground source heat pump systems (GSHP), exploiting shallow geothermal energy, have been receiving increasing interest for space heating and cooling due to their high energy efficiency and low greenhouse gas emissions. A core component of the GSHP system is the ground heat exchanger (GHE) that is in contact with the ground. Accurate heat transfer analysis of the GHE is vital for optimizing the design and operation of GSHP systems. Among the different types of GHEs, the vertical borehole heat exchanger (BHE) is widely used, and various simulation and design tools have been developed for BHEs in the past two decades.
  However, current BHE design methods have limitations as they rely on certain assumptions and fail to account for real ground conditions such as groundwater flow, multilayered ground conditions, and ground surface conditions. It has been proved that these conditions have a significant impact on the long-term thermal performance of vertical BHEs, and ignoring them can lead to inefficient GSHP systems. To address these issues, this thesis aims to refine the current design method by considering these ground conditions in the heat transfer analysis for BHEs.
  The refined BHE design method takes into account non-uniform heat transfer rates in multilayered ground conditions. The variation in heat transfer rates is influenced by inhomogeneous ground thermal properties and the presence of groundwater flow. When groundwater flow is absent, conventional solutions assuming uniform heat transfer rates may overpredict ground temperatures in certain layers by up to 50.82%. The effect of inhomogeneous ground thermal properties further strengthens the variation in heat transfer rates and temperatures along the borehole. Additionally, when groundwater flow is present, the heat transfer rate in the aquifer can be approximately 21.69% higher than the average rate. The influence of the aquifer was examined, showing that heat transfer in the aquifer strengthens with an increase in flow velocity. The study also examines the influence of ground surface conditions, particularly in the case of short boreholes over a long period of time. The difference in boundary conditions may lead to an underestimation of up to 26.3% for a 15-meter BHE after about 35.8 years. The refined design method considers the synergistic effect of the circulating fluid and the ground surface condition in analytical solutions, extending the impact of the ground surface boundary to greater depths. For example, at the bottom of a 70-meter borehole, the underestimation caused by using a constant ground surface temperature remains significant at 6.85%.
  In summary, the refined BHE design method offers improved computational time and accuracy, making it a promising tool for designing vertical BHEs. It enhances the understanding and capabilities of current design methods, leading to more effective and efficient applications of shallow geothermal energy in GSHP systems.

• Subjects / Keywords

  • ground source heat pump
  • borehole heat exchanger
  • Line heat source

• Graduation date

  Spring 2024

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-p621-ej28

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.